1. Technical Field of the Invention
The invention relates to a tibial component which comprises an anchor portion for being fixed in bone; and to a tray for a sliding plate of polyethylene, the tibial component cooperating with a femoral component which rests on the sliding plate by curved articulation surfaces.
2. Prior Art
A knee joint endoprosthesis of the generic type has been described for example in P 41 02 509 A1.
Fundamentally, it is the object of a knee joint replacement to re-establish the function of the joint as far as possible by an endoprosthesis. In doing so, special consideration must be reserved for the kinematics and biomechanics involved and their principle of sliding-rolling motions. A knee joint prosthesis must be able to copy such motions and to absorb the strain by flexion, extension and rotation without any damages to the bone bearing and capsular ligament system.
To this end, endoprostheses have been known in which the tibial plateau component and femoral component are coupled, and endoprostheses of the generic type in which there is no such coupling. These bi-condylar prostheses make use of concepts that allow a selection of varying degrees of freedom in the motion of the polyethylene sliding components in relation to the tibial components. They include rigidly fixed sliding plates, rotating platforms, or platforms rotating by a translatory degree of freedom (anterior-posterior), and freely floating platforms. Given today's operating-room conditions, substantial competitive advantages reside in a reduced number of instruments and implants and in as free as possible an implementation of intraoperative decisions, these decisions including individually optimal compromises between mobility and stability. A fixed polyethylene sliding plate on which the femoral component rest offers maximal joint stability, a freely floating plate —preferably used where stable ligaments are available —permits a maximum of mobility.
Shaping the bi-condylar femoral and sliding component on which the femoral component rests must allow for the fact that highly congruent medial and lateral articulation surfaces will reduce the pressure load on polyethylene by offering an enlarged bearing area, but may restrict the degrees of freedom of the natural sliding-rolling motion of the joint of axial tibial rotation, this leading to high shearing forces and torsional stresses in a fixed sliding component. Undesired consequences consist in premature implant loosening in addition to high local strain, deformation and wear of the polyethylene sliding component.